5.1.1 Communication and homeostasis 

All living organisms need to maintain a certain set limited of conditions inside their cells—including suitable temperature, pH, aqueous environment that keeps substrates and products in solution, and freedom from toxins and excess inhibitors. Environmental change is stimulus, and the way organism changes its behaviour or physiology is the response.

Communication systems are needed to maintain the constant internal environment in a changing external environment. As the external environment changes, it places stress on living organisms. For example, a cooler environment will increase heat loss. In order to survive these changes must be monitored and the organism must change its behaviour and physiological process to reduce the stress. This occurs through the hormonal system. More immediate changes in the external environment require quicker response, and thus a quicker communication of the stimulus through the neuronal system.

Communication systems allow organisms to coordinate their activities in order to respond to a changing internal environment. The metabolic processes of cells alter the internal environment, reducing the concentration of substrates, and increasing the concentration of products, which may be toxic. In order for the metabolic processes to be maintained, these changes must be offset, requiring detection of the changing conditions and communication between cells to offset the changing internal environment. For example, if carbon dioxide levels increase in the blood, it lowers the pH of the tissue fluid, which can disrupt the action of enzymes. Communication between chemoreceptors in the blood, the brain, and the gaseous exchange system are required to offset this change.

As multicellular organisms cells are more efficient than single-celled organisms as they are differentiated to perform their various functions such as gaseous exchange, with groups of specialised cells performing a specific function forming tissues and organs, a good communication system is needed to ensure that different organs working together to perform the same function, such as excretion of waste work together effectively even if they are not adjacent to each other. For example, the receptor may be far away from the effector.

A good communication system, then, will cover the whole body, enable cells to communicate with each other, enable specific communication, rapid communication, and short and long term responses.

The two major systems of communication that work by cell signalling between distant cells are the neuronal system and the hormonal system.

The neuronal system is a network of interconnected neurons that signal to each other across synapse junctions. The neurones can conduct a signal very quickly and enable rapid responses to stimuli that may be changing quickly.

The hormonal system uses the blood to transport chemical signals. Cells in endocrine glands release a hormone directly into the blood and it is transported throughout the body, but it only recognised by target cells with the complementary cell-surface receptors to the shape of the signalling molecule. The hormonal system enables longer-term responses to be coordinated.

Between adjacent cells gap junctions may facilitate the movement of small molecules from cell to cell in a tissue, such as those in the heart. Contact dependent communication may occur between signalling molecules in the plasma membrane of the signalling cells and receptor proteins present in the plasma membrane of the target cell. Alternatively, paracrine cells secrete signalling molecules which diffuse locally through the extracellular fluid. The molecules are quickly broken down, so the impact is confined to adjacent cells with the complementary receptor site in the cell-surface membrane.

Homeostasis is the maintenance of a constant internal environment despite changes in external and internal factors—CO2 concentration, body temperature, blood glucose, salt, and water potential, as well as pressure must be maintained within a small range of variables.

This involves negative feedback, whereby a deviation from the optimum is detected and a mechanism reverses the change. In order for this to occur, the change in the internal environment must be detected by sensory receptors, such as temperature, osmo, baro, or chemoreceptors. Then this must be communicated via the hormonal or neuronal system, which transmit a signal via a coordination centre, usually the brain, to effectors which bring about a response, Examples of effectors include liver or muscle cells. When the effector brings about a response that reverses the initial change in conditions, the system moves closer to the optimum, this is detected by sensory receptors and the stimulus is reduced.

Conversely, positive feedback is where occurs where a deviation increases the original deviation, so is a mechanism whereby a change is increased. These systems are more rare in the body, but are used to effect larger changes over a short period, For example, during pregnancy the dilation of the cervix causes the posterior pituitary gland to release oxytocin which increase uterine contractions which stretch the cervix more, causing more oxytocin to be secreted, increasing the dilation of the cervix. The action of neurons also involves positive feedback with an influx in sodium ions depolarising the cell, causing voltage-gated sodium ion channels to open.

Ectotherms are not able to control their body temperature using their metabolism; they rely on external sources of heat and their body temperature fluctuates with the external temperature.

However, using a variety of behavioural mechanisms, some ectotherms are able to control their body temperatures within acceptable ranges. Many ectotherms bask in the sun to gain heat directly from the sun, or change the orientation of their body to increase the surface area

exposed to the sun during colder parts of the day, and orientate it away from the sun at the hottest parts, as locusts do. This can also be achieved through changing the size of the body through curling into a ball, or increasing the size of the ribcage, as some lizards do. Increasing the rate of breathing when it hot allows water to evaporate from the tracheal system, cooling the body. Some use burrows or crevices between rocks which have a more stable air temperature to keep cool in the hotter parts of the day. Ectotherms increase their body temperature through conduction by pressing their bodies against warm ground.

They can use movement to increase their metabolic rate. For examples, moths and butterflies vibrate their wings before they take flight and iguanas will contract their muscles and vibrate increasing cellular metabolism. Some also have physiological adaptations. For example, Lizards living in colder climates tend to be darker colours, and some can control their heart rate.

Endotherms control their body temperature within a small range of variables, largely independent of external temperatures, using their internal exothermic metabolic activities to keep them warm, and energy-requiring physiological responses to cool them down. The interaction between sensory receptors, the autonomic nervous system and effectors helps ectotherms maintain a stable core body temperature despite a changing external environment.

The thermoregulatory centre in the hypothalamus monitors blood temperature, detecting changes in the core body temperature. However, information from peripheral temperature receptors in the skin and muscles allow the hypothalamus to respond more quickly. The physiological responses of endothermic thermoregulation are the result of homeostatic mechanisms using negative feedback control from the hypothalamus. The heat loss/gain centre is activated when the temperature of the blood changes and send impulses through the autonomic motor neurons to effectors in the skin and muscles.

Behavioural

Endotherms also use behavioral responses to temperature changes. If too cold, these include basking in the sun, orientating the body to increase surface area exposed to the sun, pressing themselves against warm surfaces, exercise to increase metabolic rate, rolling into a ball to reduce surface area, and remaining dry. If too hot, these include bathing in water or mud, digging burrows or finding shade to reduce exposure to the sun, orienting the body to reduce surface area exposure to the sun, remaining inactive, and spreading out the body, and licking the skin or fur to use evaporation to help cool.

Physiological

Being the peripheral and the largest organ, the skin has a number of physiological responses. If it is too hot, sweat glands secrete fluid onto the skin surface and as it evaporates heat from the blood is used as the latent heat of vaporisation; hair and feathers lie flat to reduce insulation and allow greater radiation, and vasodilation of arterioles and precapillary sphincters directs blood closer to the surface of the skin so more heat can be radiated. If the body is too cold, less sweat is secreted to reduce heat lost in evaporation, hairs and feathers stand erect to create a layer of insulative air, and vasoconstriction of arterioles and precapillary sphincters diverts blood away from the skin, reducing the amount of heat radiated from the body. The respiratory rate can be controlled in the liver, releasing more heat when the body is too cold, and less when too hot. When the body is too cold, the rapid contraction of skeletal muscles causing shivering, and this generates metabolic heat from the exothermic reactions.

This allows endotherms to maintain a fairly constant body temperature whatever the temperature externally and remain active even when external temperatures are low giving them an advantage as, and from, predators. Also, they can inhabit colder parts of the planet. However, it means they use a significant part of their energy intake to maintain their body temperature, need more food as a result, and use for growth a lower proportion of the energy and nutrients gained from food. They may also overheat in hot weather. Ectotherms, on the other hand, use less of their energy in respiration, more of the energy and food can be converted to growth, they need to find less food, and they can survive long periods without food. However, they cannot be active at low temperatures, which can be a predatory disadvantage.